Einstein’s early expositions of relativity placed considerable emphasis on a small number of principles that played an important role guiding his work to develop a general theory of relativity. This talk will focus in particular on the role of the equivalence principle of 1907 and “Mach’s principle,” which Einstein first articulated explicitly in 1918.

In each case I will trace their prehistory to the research of Ernst Mach, showing that in addition to his well-known critiques of Newtonian ideas and the bucket experiment, Mach’s approach had been shaped by physiological research on the perception of motion (importantly inspired by railways and mine lifts).

Yet if Mach’s work helped Einstein focus on a small number of difficult thought experiments, what Einstein made of them was distinctive, and his understanding of each – and the principles he associated with them – changed considerably as he developed his approach to gravitation and the metric field.